A photoelastic modulator (PEM) is an instrument that is used for modulating the polarization of a beam of light. A PEM employs the photoelastic effect as a principle of operation. The term "photoelastic effect" means that an optical element that is mechanically strained (deformed) exhibits birefringence that is proportional to the mount of strain induced into the element. Birefringence means that the refractive index of the element is different for different components of polarized light.
A PEM includes an optical element, such as fused silica, that has attached to it a transducer for vibrating the optical element at a fixed frequency, within, for example, the low-frequency, ultrasound range of about 20 kHz to 100 kHz. The mass of the element is compressed and extended as a result of the vibration.
The compression and extension of the optical element imparts oscillating birefringence characteristics into the optical element. The frequency of this oscillating birefringence is determined by the length of the optical element and the speed of the transducer-generated longitudinal vibration or sound wave through the material that comprises the optical element.
The effect of the oscillating birefringence of the PEM on a linear-polarized monochromatic light wave is to vary over time the phase difference between the orthogonal components of the light that propagates through the optical element. This phase difference is known as retardation or retardance and can be measured in terms of length, waves (for example, quarter-wave, half-wave), or phase angle.
The accurate measure and control of retardation (by precise detection of the polarization of the PEM output light) has numerous practical applications. Certain applications demand polarization measurement sensitivity levels on the order of 10.sup.-6.
The optical elements of some PEMs are shaped as rectangular bars. When laser light is directed via a surface-normal incidence angle through a rectangular optical element, most of that incident laser energy passes through the optical element. A small amount of the light, however, is internally reflected before exiting the element.
For instance, if one considers the beam that propagates directly through the element as the primary beam, a secondary beam that is twice internally reflected will also exit the optical element after traveling an optical path that is longer than that of the primary beam. That path (hence the path difference between the primary beam and the secondary beam) is a function of the thickness of the optical element. Since the thickness of the modulated optical element varies sinusoidally at the modulator reference frequency, an optical. element thickness change of the order of a wave length of the laser light will cause the path difference to amount to an interference condition that continuously varies from constructive interference to destructive interference. The interference can be observed by a detector as an intensity modulation occurring at the modulator frequency or one of its harmonics.
The interference attributable to the modulated secondary beam is referred to as modulated interference effects. The modulated interference effects may, in some applications, easily overpower the subtle polarization effects that are intended to be detected in a given application that uses a photoelastic modulator with laser light. That is, the interference occurs at precisely the frequency of the polarization signals sought to be detected and analyzed.
In the past, anti-reflective coatings have been used to reduce modulated interference effects. This solution is not completely satisfactory because, for example, since all of the internal reflection may not be eliminated by the coating, modulated interference effects for applications requiring highly sensitive measurements will still be detected.
This invention is directed to a photoelastic modulator system that substantially eliminates the modulated interference effects that occur when the photoelastic modulator is used for modulating laser light. Anti-reflective coatings are not applied.